1. Field of the Invention
This invention relates generally to disk drive servo control systems and, more particularly, to track code systems used by digital disk drive servo control systems to determine location of a read/write head relative to disk tracks.
2. Description of the Related Art
In conventional computer data storage systems having a rotating storage medium, data is stored in a series of concentric or spiral tracks across the surface of the disk. The storage medium can comprise, for example, a disk having a surface on which a magnetic material is deposited, such as conventional magnetic disks or magneto-optical disks. The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from the disk surface by a magnetic head transducer suspended over the disk surface that can detect the variations in magnetic orientation as the disk rotates relative to the magnetic head at thousands of revolutions per minute and generate a fluctuating data signal.
Conventionally, the magnetic head is mounted on a disk arm that is moved by a servo. A disk drive servo control system controls movement of the arm across the surface of the disk to move the head from track to track and, once over a selected track, to maintain the magnetic head in a path over the centerline of the track. In a track seek operation, the magnetic head is moved over the disk to a desired one of the tracks. To accurately position the magnetic head at the desired track, it is necessary to determine the track number beneath the head as the disk rotates and the head is moved across the disk.
In general, the tracks of a data storage disk include track identification information fields containing data that is pre-recorded in the tracks. The track identification information fields include, for example, a track address field and a servo burst pattern field. The track identification information can be provided either as part of a dedicated servo system or as a sector servo system. In the dedicated servo method, the entire surface of one disk is recorded with track identification information. A dedicated servo magnetic head is positioned over the dedicated servo disk surface in a fixed relationship relative to one or more data read/write heads positioned over data disk surfaces. The position of the dedicated servo head is used to indicate the position of the data read/write heads. In the sector servo method, each disk surface includes both track identification information data and digital data. The track identification information and the digital data are both read by the same magnetic read/write head. The tracks on a sector servo disk surface are divided into radial sectors having short sector marker, track address, and servo burst pattern fields followed by a longer digital data field. The data in the sector marker field indicates to the read/write head that position information immediately follows in the track, while the data in the track address field indicates the track number, or address code, of the track.
The dedicated servo method is most often used with multiple disk data storage systems, because a dedicated servo system for a single disk application would use one-half of the available disk surface area for track identification information and therefore would not be especially efficient. The sector servo method is more efficient than the dedicated servo method for low profile drives with fewer disks in the configuration, because a single read/write head can be used to obtain the track identification information and to read and record digital data from the disk and also because less of the disk surface area is used for track identification information. As users demand greater storage capacities from disk systems, manufacturers provide less and less disk area for track identification information by decreasing sector length and track width. To obtain the same amount of track identification information in less disk area, the information must be recorded at higher and higher frequencies. The higher frequencies increase the difficulty of reading the track identification information.
Conventionally, the track address code data is encoded on a disk as a gray code, known to those skilled in the art, to assist in the correction of read errors due to off-track detection of adjacent track address data as a track identification information head is moved across the disk. The track address is encoded as a low frequency analog signal. The track identification information head, which can be the same head used for reading the digital data for a sector servo disk or can be a dedicated servo head, detects the track address data and produces an analog signal that is sent to a preamplifier. An automatic gain control circuit typically receives the preamplified signal and produces a signal with reduced dynamic range, which makes the signal easier to process and can thereby reduce errors. The analog signal is demodulated with peak detection circuitry to determine the ones and zeroes that specify a binary track address and such track data is provided to the disk drive servo control system circuitry to provide information on the track from which the servo position information was read.
A phase-lock-loop (PLL) circuit is usually required for high-frequency synchronous detection of the gray code track address information on the disk. The PLL helps reduce phase jitter, variations in signal amplitude, and off-track noise. The synchronous detection of the track address requires a synchronization field in the servo information areas of the disk to synchronize the PLL operation with the data storage device system clock. Unfortunately, the synchronization field reduces the disk area available for recording of data. In addition, the PLL can :introduce processing errors that can require additional compensating circuit components, complicating the design and construction of the track address decoding circuitry.
In some disk drive data storage systems, the digital data is demodulated from the disk using digital processing methods. In such systems, separate analog and digital processing channels must be provided, so that the signal from the track identification information fields is provided to art analog data processing channel and the signal from the digital data fields is provided to a digital data processing channel. After the track address data is determined, it is known to digitize the track address data and provide it to digital control circuitry, but the track address demodulation still is performed by the analog circuitry. Providing separate data channels increases the complexity of the disk drive circuitry and increases the cost of producing disk drives.
The desire for increased storage capacity, resulting in what are commonly referred to as high density disk drive systems, also has resulted in new read/write head technologies. For example, magneto-resistive (MR) read/write heads are becoming more common because they permit reading of data at relatively high frequencies even with lower disk rotational velocity. The higher frequencies permit track identification information data, servo burst pattern data, and digital data to take up less disk space, increasing disk capacity. Unfortunately, the nonlinear response characteristics of MR read/write heads result in strong second-order harmonics in the digitized signal that can introduce extra errors in the resulting demodulated track address data, which can cause mistracking of the read/write head.
From the discussion above,, it should be apparent that there is a need for a disk drive with a track address encodement and detection scheme that reduces overall circuit complexity and accommodates narrow data tracks. The present invention fulfills this need.